Improvement in screw-taps



W. TUCKER.

improvement in Screw-Taps.

No. 129,376. Patented luly16,1a72.

Witnesss. Men-tor wmf@ Vamo@ ,z f -f /--7 -v UNITED STATES PATENTOFFICE.

WILLIAM TUCKER, OF FISKEDALE,MASSACHUSETTS, ASSIGNOR TO HIMSELF ANDEMORY L. BATES, OF SAME PLAGE.

IMPROVEMENT IN SCREW-TAPS.

Specification forming part of Letters Patent No. 129,376, dated July 16,1872.

Specification of an Improved Screw-Tap, invented by WILLIAM TUCKER, ofFiskedale, in the county of Worcester and State of Massachusetts.

The invention relates to that class of screwtaps in which the sectionsor cuts are separated and formed by longitudinal grooves, which alsoserve to discharge the cuttings or chips.

These have heretofore beenmanufactured of two general forms -the iirstcircular in cross-section; the second having sections or cuts depressedfrom front to rear to give saliency to the cutting-edges. The secondform is free from the objections to the iirst, but is itselfobjectionable in two very important respects: First, the open space atthe heel of each section permits and tends to cause the chips to enterand wedge between the tap and the cut thread, injuring the latter andendangering the breakage of the tap. Second, the tap is liable todeection, owing to its few points of contact.

This invention consists in shortening the radiuses of the tap ordepressing the threads between the cutting-edge and the back or heel ofeach section or cut; the object being to retain the prominency ofthethreads at the heels so as to scrape the chips into the grooves' inturning the tap backward, with the advantage also of multiplied pointsof contact for insuring a straight cut and to obtain the desiredsaliency of the cutting-edges.v The improved tap is adapted to be madeby machinery, and with thread of full depth at the depressed points.

In the accompanying drawing, Figure lis an elevation of a tapillustrating the invention. Fig. 2 is a transverse section of the sameon the line z z, Fig. 1, and also a diagram illustrating the improvedconstruction.

In carrying out my invention, I make a solid l steel tap, A, withcircumferential screw-thread a, increasing in height from the enteringpoint, and interrupted by longitudinal grooves b to form cutting-facesor points 1 and to discharge the chips, as heretofore practiced. To giveproper saliency to the cutting-edges or points 1, and at the same timeinsure the free with drawal ot' the tap and proper and sufcient pointsof contact, I form the thread or threads a at different distances fromthe axis 2, or with radiuses of dilerent lengths in a given transverseplane, as illustrated in Fig. 2. The radiuses of a circuit of the threadare greatest at the cutting-edges 1. From the cutting-edge of eachsection the radiuses shorten to a point, 3, at about the middle of thesection, and then lengthen to the back edge or heel 4., where the radiusis equal to that ofthe cutting-edge less the decrease incident to thepitch of the thread in the slightly-conical tap. In other words, theradiuses of the tap are shortened or the thread depressed between thecuttingedge and heel ot' each section or cut without reducing theprominency of either of said points,

vand without varyin g the depth of the thread.

The prominent heels 4 touch within the bore in withdrawing the tap andscrape any chips which may have adhered into the grooves b, and alsoform additional points or lines of contact, insuring a straight cut. Thecentrally-depressed faces of the respective sections may be cut in anordinary screw-cutting lathe by using ecentric centers 5, as illustratedby dotted lines in Fig. 2; or they may be otherwise produced.

I claim as new herein- A screw-tap with the thread depressed or radiusesshortened between the cutting-edge 1 and heel toi each section or cut,the radiuses at the heels being equal to those at the cuttin g-ed gesless the decrease incident to the pitch of thc thread, substantially asdescribed.

WILLIAM TUCKER.

Witnesses:

B. L. FROST, EMoRY L. BATES.

